In casting strip according to the two-roll method the smelting space on the front sides of the casting rolls is normally sealed with side plates which support a ceramic insert forming the actual seal. This ceramic is displaced axially during the casting process in the direction of the casting rolls in order to compensate on the one hand for the removal of the ceramic material caused by the abrasion of the casting rolls and on the other hand for the wear of the insert which is brought about as a result of abrasion by the strip edge forming in the casting gap and a chemical reaction.
In practice, it has been shown that the wear caused by the strip edges on the inserts of the side plates has a decisive influence on their service life and hence on the entire economy of a strip casting plant of the type concerned. If the wear caused by the strip edges is not counteracted in time, gap-like grooves are formed in the inserts as the operating time increases, which grooves depart from the surface of the respective insert assigned to the casting gap and extend along the contact region against which the circumference of the casting rolls bears on the peripheral surfaces of the insert. According to the strength of the melt, which increases in the direction of escape from the casting gap, the width and depth of the grooves increase here in the direction of the narrowest region of the casting gap on which the metal shells solidified on the casting rolls are pressed together. Due to extrusion effects the gap-like grooves are able to work into the regions of the insert ceramic lying between the front faces of the casting rolls and the contact faces of the inserts assigned to them. The gap-like grooves enlarged in this manner then have a T-shaped cross-section. Melt penetrating the T-gap-like grooves may solidify on the front faces of the casting rolls. The solidified melt pieces, also called “T-edges” in the technical jargon, are entrained by the casting rolls and may then considerably distort the results of casting operation and casting result.
If such grooves are formed on the insert molten metal may penetrate these grooves and solidify in them. Such solidifications vary on the strip escaping from the casting gap in that the edge of the strip, viewed in cross-section, has a “T-shape” where thin webs of solidified metal are formed on the edges in the region of the transition of the narrow sides to the longitudinal sides of the strip cross-section, which webs can lead to considerable problems in the casting operation and have a decisively negative influence on the quality of the cast strip obtained. For example, the metal penetrating between the insert and the respective casting roll is cooled in the casting gap far more intensely than the volume of metal located centrally in front of the insert. Whilst the solidified metal penetrating between the respective casting roll and the insert is entrained over the width after escaping from the casting gap due to strip shrinkage, the central section of the strip sags due to the influence of gravity after escaping from the casting gap. Consequently, stresses are generated in the strip material not yet solidified, which bring with them the risk of formation of longitudinal cracks in the finished cast strip. In extreme cases the T-edges separated as a result of this and rotating with the casting rolls can result in an interruption in casting.
WO 2004/000487 discloses that the formation of grooves can be counteracted by moving the side plates with their inserts after reaching a stationary casting process, in a first interval of time in the axial direction of the casting rolls against the front sides of the casting rolls, then in a second interval of time in a direction that is parallel to the direction of conveying, with which the cast strip leaves the cast strip. In this case the first interval of time of the axial movement can at least partially overlap the second interval of time of the movement that takes place parallel to the direction of conveying of the strip. However, the first interval of time should in each case always commence before the second begins to ensure perfect grinding of the side plate inserts. The feed, with which the side plates are moved with their inserts in the direction of conveying of the cast strip, is in this case within the range of 50 mm per hour, preferably within the range of 1 mm to 30 mm per hour of casting time.
Practical tests have shown that although the risk of formation of grooves and the accompanying problems can be reduced by the procedure disclosed in WO 2004/000487, it was shown that this reduction was not yet sufficient to avoid the formation of grooves occurring to a greater extent under the conditions prevailing in practice with sufficient certainty for the period of operation.
In addition to the prior art explained above, DE 100 56 916 A1 discloses a further method for casting a metal strip on a two-roll casting machine whose casting gap, formed between the two casting rolls, is also sealed on its short sides by side plates supporting an insert of refractory material. Here, the side plates are displaced along an approximately vertical plane in the direction of casting to optimize the sealing, an oscillating vibration being superimposed upon the displacement. The purpose of this measure is to avoid particle accumulations, so-called “crusts”, adhering to the side plates, solidifying at an early stage. These crusts may lead, subject to local limitations, to accumulations on the side plates, which extend into the metal present in the casting gap and already solidified in the regions adjacent to the casting rolls, but between them still in the molten state. Problems of this nature occur particularly in two-roll casting plants in which the temperature and flow conditions, in particular, are unfavorable in the side plate region. If the particles adhering to the side plates become detached there is a local increase in strip thickness in the edge region. Because of the massive widening of the strip that is still possible during strip casting in the condition after the two-roll process, there may as a result be an extreme pressure increase on the laterally limiting inserts of the side plates, this increase provoking a fracture of the same, particularly in the lower region. These fractures of the lower insert sections, and also fractures caused by other effects, generally result in the lower insert edge displaced upwards and newly produced by the fracture being located below the process-dependent strip shell contact point, the so-called “kissing point”. Equally inadequate positioning of the lower insert edge may also be caused by unfavorable process parameters which initiate a displacement of the kissing point under the lower insert edge. These relationships result in an undesirable so-called “drop edge formation”, where the material, still in the molten state, is forced out of the casting gap or runs out directly from it in the extreme case due to the strip widening force.
In practice, this risk is counteracted by lowering the side plates for a short time where the positioning of the lower edge is correspondingly inadequate. The problem of the formation of gap-like grooves, which occurs, in particular, in highly developed and extremely efficient two-roll casting machines, is not discussed in DE 100 56 916 A1.